This invention relates generally to hard disk drive memory storage devices and, more particularly, to a time domain solution to self-servo writing disk files.
Conventional hard disk manufacturing techniques include writing servotracks on the media of a head disk assembly (HDA) with a specialized servowriter instrument. Laser positioning feedback is used in such instruments to read the actual physical position of a recording head used to write the servotracks. Unfortunately, it is becoming more and more difficult for such servowriters to invade the internal environment of a HDA for servowriting because the HDAs themselves are exceedingly small and depend on their covers and castings to be in place for proper operation. Some HDAs are the size and thickness of a plastic credit card. At such levels of microminiaturization, traditional servowriting methods are inadequate. Other benefits of self-servo writing are disclosed, for example, in U.S. Pat. No. 5,907,447.
Conventional servo-patterns typically comprise short bursts of a constant frequency signal, very precisely located offset from a data track's center line, on either side. The bursts are written in a sector header area, and can be used to find the center line of a track. Staying on center is required during both reading and writing. Since there can be between a few to several hundred, or even more, sectors per track, that same number of servo data areas must be dispersed around a data track. These servo-data areas allow a head to follow a track center line around a disk, even when the track is out of round, as can occur with spindle wobble, disk slip and/or thermal expansion. As technology advances provide smaller disk drives, and increased track densities, the placement of servo data must also be proportionately more accurate.
Servo-data are conventionally written by dedicated, external servowriting equipment, and typically involve the use of large granite blocks to support the disk drive and quiet outside vibration effects. An auxiliary clock head is inserted onto the surface of the recording disk and is used to write a reference timing pattern. An external head/arm positioner with a very accurate lead screw and a laser displacement measurement device for positional feedback is used to precisely determine transducer location and is the basis for track placement and track-to-track spacing. The servo writer requires a clean room environment, as the disk and heads will be exposed to the environment to allow the access of the external head and actuator.
FIG. 1 shows a self-servo writer machine in which servo patterns are written from the inner diameter (ID) to the outer diameter (OD), i.e., one track at a time. FIG. 1 shows a series of points in time as the disk rotates at one servotrack position over servo data in subsequent propagation burst sectors. Sectors are defined herein as a group of radial propagation bursts from which a single position measurement is obtained on each disk rotation at each sector. At time T0, a section of the disk drive 400 is shown of the disk surface A with the write head 403 positioned to write another propagation radial servo burst at 406 in this sector as the disk rotates. The direction of track motion is horizontal in the illustration. The read head 404, offset from the write head by a distance 405, will pass over the previously written radial propagation bursts 412, 413, 414, and 415. In such a configuration, track shape errors can propagate or expand with each track written. Fixing errors is a signal processing problem.
Heretofore errors are treated as repetitive waveforms. For example, U.S. Pat. No. 5,907,447 discloses the use of Fast Fourier Transform (DFT) algorithms to keep self-propagated servopattern track shape errors from growing during the self-servowriting process. Errors in track shape corresponding to frequencies at which the magnitude of the closed loop response exceeds unity will be amplified on subsequent tracks leading to exponential growth of track shape errors. This exponential growth occurs for both systematic (such as write width modulation) and random errors (from TMR (Track Mis-Registration)). Thus, the servo closed loop response corresponds to a step-to-step amplification factor. One solution provided in the '447 patent is to use servo loop parameters that make the magnitude of the closed loop response less than unity at frequencies equal to integer multiples of the rotation frequency. In addition, a feedforward correction is applied to the amplitude burst propagation pattern in order to prevent the propagation servo loops from attempting to follow the track shape error on the next step. The correction may be applied to the amplitude burst propagation pattern simply by modifying the reference amplitude values used by the servo loop before stepping to reflect the position error signal as detected during the write. By pre-compensating the reference amplitudes to account for the known position error during the write, the servo loop will register no error as it follows a smooth trajectory.
As a frequency domain solution, FFT algorithms tend to be complex and difficult to understand. Moreover, such complex algorithms require a significant amount of computational power that often cannot be performed by the HDC (hard disk controller) of a HDD (hard disk drive).